It has long been known that high finesse optical cavities amplify optical loss processes occurring between the cavity optics. This cavity amplification ultimately allows highly sensitive measurements of such optical losses to be achieved, e.g., for detecting and determining the concentration of specific chemicals of interest, both at extremely low levels (˜ppb) and with very short response times (˜μs). Absorption spectroscopy measurements of this kind are useful for a variety of applications, including pollution monitoring, toxic chemical detection, process control monitoring, off-gas monitoring, trace gas analysis, purity analysis, and medical diagnostics (e.g., breath analysis).
Several methods for using optical cavities for these purposes have been described. One of the most common cavity-based spectroscopic methods is known as cavity ringdown spectroscopy (CRDS). Radiation is injected into an optical cavity, either by a single laser pulse or by an abruptly interrupted continuous-wave (CW) laser, which has been chosen to match an absorption wavelength of an atomic or molecular species of interest. A photodetector measure total intra-cavity loss by observing the exponential decay over time of the output intensity following the radiation injection. The time constant of the decay depends upon all losses in the cavity, including losses due to chemical absorption, with stronger absorption producing a faster decay rate. Intrinsic losses can be isolated by measuring the decay rate in the absence of any chemical absorbers.
Other methods, such as integrated cavity output spectroscopy (ICOS) and noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS), use cavity transmission properties to gauge the intra-cavity loss. ICOS is similar to CRDS, but does not involve pulsing or blocking of the laser light or measuring the cavity decay rate. Instead, continuous light injection is used, and the intra-cavity intensity builds to a saturation value determined by the cavity mirror reflectivity and the sample absorption. In these cases, the intrinsic cavity loss may be determined separately, by suing CRDS for example, to obtain quantitative absorption intensity data. Light trapped in the optical cavity passes through the absorbing sample many times, so the observed amplification of the absorption signal is very large (typically on the order of 100 to 10000).
Most of these methods in some way manipulate the optical resonances that arise in the cavity due to the periodic boundary conditions imposed on the intra-cavity electric field by the mirror surfaces. These resonances, which are interferometric in nature, comprise the general subject of Fabry-Perot theory. To precisely control the resonances generally requires complex and expensive instrumentation and hardware, and places extreme constraints on the overall stability of the apparatus.
In a typical version of ICOS, the system may be dithered, in which the laser's wavelength is rapidly modulated over a frequency spacing containing several cavity modes, or in which one of the cavity mirrors is rapidly vibrated with a piezoelectric transducer to oscillate the cavity length, or both. This forced rapid randomization of cavity modes allows the cavity output intensity to be averaged to within ΔI/I0-10−2 or better.
More recently, two off-axis techniques, known as Off-Axis Cavity Ringdown (oa-CR) and Off-Axis ICOS (oa-ICOS), have been developed that introduce the light into the optical cavity along an off-axis light path so as to systematically disrupt optical resonances and remove the frequency selectivity of the cavity, thereby rendering it effectively a broadband device. As a result, narrowband lasers (bandwidth Δv<100 MHz) can be used without activity controlling the cavity length. This eliminates the need for expensive components such as acousto-optic modulators, piezoelectric transducers, lock-in amplifiers, etc. This design also reduces optical feedback from the cavity into the source laser, which is particularly important for the case of a distributed-feedback diode laser as the source laser. Previously, either expensive Faraday isolators or three-mirror ring-cavities have been used for this purpose. Additionally, the constraints on the overall system alignment are vastly reduced. Rather than having only one possible alignment geometry (i.e., the laser on-axis with the cavity), any of the many stable paths through the cavity can be used. This allows faster alignment routines, and lowers the sensitivity of the instrument to vibration. Coupled with a slight astigmatism (optional) of the cavity mirrors, the off-axis light path increases the beams's reentrant condition from a single pass (for standard ICOS) to almost 1000 passes, which well exceeds the coherence length of the laser light. As a result, the cavity's output intensity can be effectively averaged to within ΔI/I0˜10−4.
A drawback of the off-axis geometry is that, since none of the resonant cavity frequency modes are preferentially populated, no appreciable power build-up occurs inside the optical cavity. This results in a net reduction in the transmitted power by a factor of T/2, where T is the average cavity mirror transmission. This power reduction can be very significant, since T is on the order of 10−4 to 10−5 for mirrors typically used in such applications, meaning that in applications employing a milliwatt laser, the detector will measure only 10-100 nanowatts of power. Many potentially interesting applications of these off-axis techniques that would employ weak laser sources, such as cryogenic lead-salt diode lasers, or even non-laser sources, are not practical since the signal on the detector would be too low to be useful. Thus, a way to greatly increase the amount of optical power that can be injected off-axis into an optical cavity would improve the applicability of the oa-CR and oa-ICOS techniques.